Positioning and pinching slow proton movement in catalyst

Twisting and pinching slow a catalyst's ability to generate energy from hydrogen, according to scientists at Pacific Northwest National Laboratory's Center for Molecular Electrocatalysis. In converting hydrogen to electricity, the catalyst uses claw-like extensions, called ligands, to get needed items in place. However, the ligands opening and closing during the reaction can't be speeded up, making it the rate-limiting step; the ligands' tendency to grasp the protons a bit too tightly can also slow the reaction.

Why It Matters: Reducing our nation's reliance on fossil fuels relies on creating technologies to quickly and efficiently convert the power generated at wind farms and other sources into a use-any-time fuel, such as hydrogen molecules.  Scientists are striving to design affordable, abundant catalysts that turn energy into hydrogen and then release the energy when desired. This study, which appears in the Journal of the American Chemical Society, answers important questions about how the chemical energy in the hydrogen would be converted into electrical energy.

The team examined a nickel-centered catalyst with two ligands, six-sided molecular bits that extend off the edge of the catalyst and resemble crab claws. The specific catalyst is [Ni(PCy2NBn2H)2]2+ (PCy2NBn2 = 1,5-dibenzyl-3,7-dicyclohexyl-1,5-diaza-3,7-diphosphacyclooctane).

The catalyst's ligands snag a hydrogen molecule and move it to nickel at the heart of the catalyst. The nickel cracks the molecular hydrogen into 2 electrons and 2 protons. The electrons remain at the center and are then free to do work, just like the electrons generated by fossil fuel combustion. The ligand picks up the protons using a pendant amine, a small string of atoms containing nitrogen that dangle off the ligand.

During this process, the ligand twists between two different conformations, known as "boat" and "chair." Changing between boat and chair is the rate-limiting step in the reaction, meaning that the reaction can only go as fast as this change occurs. This proved to be an unexpected result for the team.

But, there were other surprises to be found in the data as well. When the ligand picks up the protons it uses the same pendant amine that was involved in taking the hydrogen to the center.

"Intuitively, I would have expected the molecules to completely scramble the protons," said Dr. Wendy Shaw, recipient of the U.S. Department of Energy's Early Career Award and physical chemist on this study. "That the protons moved on the same ligand every time is really interesting and could have implications for future catalysts."

Further, the team found that the pendant amine isn't always ready to give up the protons. Under certain conditions, the molecule holds on tightly to the protons, pinching them. The amines can pinch the protons so hard that they can't move.

In this study, the researchers used a complementary experimental and theoretical approach. The experimental scientists determined the conformation of the catalyst and the rates of reaction using nuclear magnetic resonance spectroscopy experiments in PNNL's Physical Sciences Laboratory. The theorists described the proton movement using complex calculations, including density functional theory and molecular dynamics simulations. The theoreticians used supercomputers at two user facilities. The team used resources at EMSL, Environmental Molecular Sciences Laboratory, and NERSC, the National Energy Research Scientific Computing Center.

"This paper is an example of what a research team can do when they are working closely together," said Dr. Morris Bullock, Director of the Center for Molecular Electrocatalysis at PNNL. "Together the experimentalists and theorists really answer the question of how the protons move."

The experimentalists and theorists, together in the Center for Molecular Electrocatalysis, are using this research as they tackle two new challenges in proton movement. First, they are conducting a more detailed examination of proton pinching. Second, the team is studying how pendant amines take the next step, moving the proton from the edge of the molecule to the environment beyond. This relay hand-off is critical for fuel cells and other alternative energy technologies.

"What we found has really helped us think about how to design new catalysts," said Shaw.

More information: O'Hagan M, WJ Shaw, S Raugei, S Chen, JY Yang, UJ Kilgore, DL DuBois, and R Bullock. 2011. "Moving Protons with Pendant Amines: Proton Mobility in a Nickel Catalyst for Oxidation of Hydrogen." Journal of the American Chemical Society. Article ASAP.

Provided by Pacific Northwest National Laboratory (news : web)

Positioning enzymes with ease

Virtually all processes in the human body rely on a unique class of proteins known as enzymes. To study them, scientists want to attach these molecules to surfaces and hold them fast, but this can often be a tricky undertaking.

Now Jinglin Fu and his colleagues at the Biodesign Institute at Arizona State University have developed a superior method for immobilizing enzymes on surfaces, deftly controlling their orientation, improving their efficiency and rendering them more stable. The group's results appear in today's advanced online issue of PLoS ONE.

Enzymes are essential for the normal functioning of cells, and are involved in tasks including cell regulation, metabolism and signal transduction. They are also necessary for muscle contraction and the transport of ions and other materials throughout the cytoskeleton.

Enzymes like amylases and proteases are central players in the digestive systems of many animals, breaking down starches and other large molecules into smaller parts that can be absorbed by the intestines. Herbivorous animals make use of the enzyme cellulose, to break down plant fiber. "No wonder enzyme function has been a topic of longstanding concern for biochemistry and medicine," says Fu.

Like other proteins, enzymes are composed of linear chains of amino acids. They can range from tens to thousands of amino acids in length. The job of the enzyme is to increase the rate of the desired reaction, without increasing the rate of undesired reactions. Here, a molecule known as the substrate interacts with a given enzyme to produce a product. Without enzymes, many reactions essential to living things could not proceed.

Such catalytic activity has also been adapted and broadly applied in the biomedical arena (especially for various diagnostic testing), as well for industrial applications ranging from photography to the brewing of beer.

Enzymes are also critical for the study of disease. Given their central role in maintaining homeostasis, any single enzyme aberration, including mutation, overproduction, underproduction or deletion can have dire consequences for health. Phenylketonuria, for example, is a disease linked with a single amino acid mutation in the enzyme phenylalanine hydroxylase. If untreated, the condition can lead to mental retardation. Malfunctioning of DNA repair enzymes is associated with a number of forms of cancer.

To properly study enzymes, particularly their catalytic activity, it is necessary to fix them in place on a surface. While researchers have used several techniques for enzyme immobilization, existing methods suffer from several shortcomings. Enzymes need to be properly oriented on the surface with respect to the molecule they are catalyzing in order to work properly. The non-specific binding of proteins can contaminate the reaction and lower or block its efficient progress. Finally, proteins are prone to becoming unfolded and deactivated over time—a process known as denaturation.

In the current study, Fu first generated a high-density array of peptides on a glass slide, each peptide composed of 20 randomly assembled amino acids. A specific enzyme, ß galactosidase, was then screened against this array. This method identified two peptides that covalently bound to the enzyme with high affinity, and these were used for the subsequent experiments.

When compared with low-affinity binding peptides and with preexisting surface immobilization techniques, the group found that the high affinity peptides not only were more effective at holding the enzyme in its proper orientation on the slide, they also produced higher specific activity in the enzyme. The enzyme was also less subject to denaturation, compared with controls.

In a further refinement of the technique, the group created mutations of the high affinity peptides, by deleting a single amino acid along the peptide's length and replacing it with a different amino acid. This procedure was repeated with all 20 amino acids in the peptide chain, with the resulting mutations once more screened against the ß galactosidase enzyme. The technique, known as single-point variant screening, improved both the binding affinity and specific activity of the bound enzyme.

"This development gives us a new tool, both for enhancing the function of surface bound enzymes, which are of ever-increasing importance to industry, and also for studying the interactions between multiple enzymes in a metabolic pathway," said Neal Woodburry, a co-author of the PLoS ONE study.

Provided by Arizona State University (news : web)